1. Field of the Invention
The present invention relates in general to an apparatus for converting outline data representative of the outline of characters such as letters and symbols and other visible representations, into dot data representative of image dots to be reproduced so as to collectively define the visible representations in a matrix of dots. More particularly, the invention is concerned with such a data converting apparatus capable of converting the outline data into dot data such that the outlines of the reproduced characters have smoothed curved portions.
2. Discussion of the Prior Art
Visible representations or characters such as letters, symbols and numerals represented by character data are printed, displayed or otherwise reproduced according to the character data suitably processed by a computer. In this field of data processing, it is widely practiced to use dot data indicative of whether an image dot is to be formed at each of picture elements which are the smallest part of a picture image and which determine the resolution of the reproduced image. If a batch of dot data representative of all the characters available for reproduction is prepared and stored in a character data memory, the memory should have an extremely large storage capacity. It is therefore desirable to store in the character memory a batch of outline data representative of the outlines of the characters, and convert the outline data into the corresponding dot data by suitable data converting means, when the characters are printed, displayed or otherwise outputted. An example of such data converting means is disclosed in Japanese Patent Publication No. 53-41017.
For converting the outline data into dot data as indicated above, the outline of each character represented by the outline data is superimposed on a coordinated pixel screen in which picture elements are defined by a plurality of pixel lines parallel to the X axis and a plurality of pixel lines parallel to the Y axis perpendicular to the X axis. The outline data is converted into dot data such that bits of the dot data are set to indicate the presence of image dots at the picture elements which are located within the outline of a character on the screen, so as to satisfy a predetermined condition. For instance, image dots are placed at the picture elements whose centers lie within the character outline.
The outline of each character consists of two or more straight or curved segments which represent one or more stroke of the character. However, the number and positions of image dots to be reproduced for a certain stroke of a character according to the dot data prepared by conversion from the outline data may undesirably vary, depending upon the positions of the outline segments defining the character stroke, relative to the pixel screen, i.e., depending upon the position at which the character in question is reproduced. This tendency is high especially for curved strokes of characters (curved segments of the character outlines).
FIG. 6 shows a letter "O" consisting of a stroke whose outline includes an outer loop 46 which consists of four curved segments. These curved segments have maximal and minimal points 11 as viewed in the x-axis and y-axis directions. As shown in the figure, image dots are formed at the picture elements corresponding to the maximal and minimal points 11, while image dots are not formed at the picture elements along the portions of the curved segments which are near the maximal and minimal points 11. This results in discontinuous or rugged external profile of the letter "O", giving an unpleasant appearance of the letter "O" as defined by the image dots formed according to the dot data prepared by conversion from the original outline data. The same phenomenon may occur for an inner loop 47 of the outline of the letter "O", which consists of four curved segments. For example, an image dot may not be formed at the picture element corresponding to each of the maximal and minimal points of the curved segments, while image dots are formed at the neighboring picture elements along the curved segments. The rugged profile of a character is perceived particularly where the width of the character stroke or strokes is relatively small, that is, where the number of the successive image dots which define the character stroke is relatively small. Even for a character having a relatively large stroke width, however, the local presence or absence of image dots at and near the maximal and minimal points of the curved segments of the stroke outline causes an unpleasant appearance of the reproduced character, as if the reproduced character was spotted by dust or dirt at its outline.